Organic semiconductors (OSs) are now controlling the electronics market in different stages of development for a wide range of applications and are expected to dominate over the next few decades. Key factors driving penetration of organic electronics in the mainstream market are their low cost for fabrication and high degree of flexibility. Since their sensational discovery in 1977 several studies have shown the possibility to modulate their properties for the specific application using chemical doping. Conceptually similar to inorganic semiconductors, chemical doping introduces impurities into the organic semiconductors which increase the density of mobile charge carriers and thus conductivity. In reality, chemical doping can involve several different mechanisms depending on both the semiconductor and the dopant. One of the first described mechanisms was the protonic acid (H) doping of polyacetylene. This was accomplished by immersing polyacetylene in an aqueous hydrochloric acid solution. Drying the polymer leaves residual H3O+ groups in the organic matrix which increase the density of mobile charge carriers by partially withdrawing electrons from the conjugated chains. The resulting p-doped organic semiconductor shows an increase in conductivity of around 10 orders of magnitude. Following this, H+ treatment has been reported as a general doping route for many conjugated polymers. Recently, several heterocyclic organic hydrides have been reported as efficient n-type dopants by releasing hydride (H−) or hydrogen atoms (H.) (P. Wei, J. H. Oh, G. Dong and Z. Bao, Journal of the American Chemical Society, 2010, 132, 8852-8853). In particular, 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzoimidazole derivatives (DMBI) can increase the conductivity of [6,6]-phenyl C61 butyric acid methyl ester (PCBM) by more than 4 order of magnitude. After the H. release, the highly energetic DMBI radicals can inject electrons in the PCBM increasing the density of mobile charge carriers in the organic semiconductor matrix. The stability of this doping mechanism depends on the rate of back electron transfer from the doped PCBM to the DMBI cation, which can be stabilized by electron-rich substituent on DMBI core. This mechanism has been effectively applied to prepare air stable organic thin film transistors. Many other organic electronic applications can benefit from chemical doping: e.g., triphenylamine-based organic semiconductors have been p-doped with Co(III) complexes in solid-state dye-sensitized solar cells (ss-DSSCs) (J. Burschka, A. Dualeh, F. Kessler, E. Baranoff, N. L. Cevey-Ha, C. Yi, M. K. Nazeeruddin and M. Grätzel, Journal of the American Chemical Society, 2011) and tetracyano-quinoline derivatives in organic light emitting diodes (B. W. DAndrade, S. R. Forrest and A. B. Chwang, Applied physics letters, 2003, 83, 3858-3860). Both applications benefit from the introduction of a p-doped transport layer since it reduces the charge transport resistance in series with the p-n junction and helps to achieve ohmic contacts. Furthermore, chemical doping also enables the fabrication of tandem structures for efficient organic solar cells using a versatile recombination contact.
However, most of the dopants reported in scientific literature have never been effectively applied in electronic device, since they are incompatible with solution-processed materials or they demonstrate weak doping effects, which makes the device unstable in working conditions. Due to the beneficial characteristics of protic ionic liquids (PILs), such as good solvation and strong acidity, solution-processable and effective doping techniques have been found using PILs.
Organic semiconductors (OSs) are commonly employed in photovoltaics, organic light emitting diodes (LEDs), and organic transistors. One family of commonly employed organic semiconductors as hole transporting materials (HTMs) are triphenylamines (also known as triarylamines) and their derivatives. These have been successfully employed in dye-sensitized solar cells (U. Bach, D. Lupo, P. Comte, J. E. Moser, F. Weissortel, J. Salbeck, H. Spreitzer and M. Gratzel, Nature, 1998, 395, 583-585). Triphenylamine derivatives often benefit from wide bandgaps and easily tunable HOMO levels, making them especially promising as HTMs for solid-state dye-sensitized solar cells (ss-DSSCs) where the light is absorbed by a sensitizing dye. However, this class of HTMs suffers from low hole mobilities, with one of the most commonly employed triphenylamine HTM (spiro-OMeTAD) having mobilities between 10−5 and 10−4 cm2 V−1s−1 in working device conditions. As a result, these materials have been doped by a variety of molecular oxidants in attempts to minimize the resistance to charge transport in the HTM component of ss-DSSCs (U. Bach, EPFL, 2000; H. J. Snaith et al., Nano Letters, 2006, 6, 2000-2003).
However, these “in situ” doping approaches have several drawbacks. It is difficult to establish the efficiency of the oxidation, and it is difficult to distinguish what degree of oxidation occurs in the initial HTM spin-coating solution compared to in the final film. The long-term stability of the oxidised product in the final device structure is often poor, or may even vary with ambient conditions such as humidity, temperature and light exposure. Moreover, these approaches to doping result in the oxidation products staying in the film, so that the final device incorporates impurities that do not serve any beneficial function and may even hinder device performance and long-term stability. As can be expected, these in-situ doping processes result in a great deal of variability in final device performance.